THE ALGAL COMMUNITY AS AN INDICATOR OF THE TROPHIC STATUS OF LAKE PATZCUARO, MEXICO

An evaluation of water quality and phytoplankton composition was carri ed out in order to determine the trophic conditions of Lake Patzcuaro (2035 m above sea level), a high altitude tropical lake. Temperatures ranged from 15 to 23-degrees-C Total phosphorus and inorganic nitrogen showed a seasonal variation; highest values coincided with the rainy season (0.48 and 2.1 mg litre-1, respectively). Dissolved oxygen range d from 2 to 7.9 mg litre-1 at the surface and from 0.6 to 7.3 mg litre -1 on the bottom, the lowest values being found in shallow zones. Aver age transparency varied from 0.62 to 1.4 m Secchi depth. Rainfall was a primary factor in seasonal variability as it influenced both physica l and biological conditions by contributing to the transport and depos ition of silt, which mixed with sinking algal cells. The composition o f the surface phytoplankton segregated along five major divisions comp rising a total of 49 species. General seasonal patterns of dominance a lternated in a sequence beginning with Bacillariophyta, through Chloro phyta to Cyanophyta. Diatoms, the dominant group from February to earl y June, included Melosira granulata, Stephanodiscus sp., Synedra sp. a nd Fragilaria sp. During the rainy season (late June to September), Mi crocystis aeruginosa, Oscillatoria sp., Anabaena sp., Merismopedia sp. , Crucigenia cuadrata, Oocystis lacustris, Selenastrum gracile, Mallom onas sp. and Tetraediella sp. were important. Melosira granulata was p resent throughout the period of study. Spatial and temporal variabilit y in both physical and biological conditions make it difficult to assi gn a specific trophic state to Lake Patzcuaro. Nevertheless, analysis of the algal community indicates a generally mesotrophic condition.